The structure of the novel cadmium phenanthroline thiosulfate poly[[(1,10-phenanthroline-kappa(2)N,N')cadmium(II)]-micro(3)-thiosulfato-kappa(3)S:S:O], [Cd(S(2)O(3))(C(12)H(8)N(2))](n), with a pentacoordinated Cd centre, is reported. It forms linear chains built up around a 2(1) axis and is isostructural with the known bipyridine homologue. The structure is also compared with a previously reported closely related monoaqua monohydrated phase, where the Cd(2+) cation is hexacoordinated. The incidence of weak C-H.O interactions in the determination of its general packing properties is discussed.